Dongkyun Kim

Relative Importance of the Different Rainfall Statistics in the Calibration of Stochastic Rainfall Generation Models

Stochastic rainfall generators are used in hydrologic analysis because they can provide precipitation input to models whenever data are not available, and their parameters are calculated so that the long-term statistics of the synthetic rainfall time series match those of the rainfall records. However, although mentioned in the literature, the relative importance of each rainfall statistic on the watershed response has not been addressed yet, and no guidance on how to account for it has been provided. In this paper, this relative importance is estimated and used to ponder each statistic differently in the calibration of rainfall generators so that it better reflects the watershed hydrology. Rainfall records of 1,249 rain gauges throughout the contiguous United States were used in the study. It was found that when synthetic rainfall time series are generated by weighting the precipitation statistics according to their relative importance, predicted runoff depths and peak flows are underestimated by 4 and 3%, respectively, whereas when they are generated by giving the same weight to all statistics, the underestimation is by 20 and 14%, respectively. These results, based on a significant number of rain gauges, confirm the benefit of weighing the statistics differently for watershed analysis. DOI: 10.1061/(ASCE)HE.1943-5584.0000453.

Observation Method for Estimating Future Scour Depth at Existing Bridges

AbstractBridge scour can cause damage to bridge foundations and abutments. Bridges with foundations that are unstable for calculated and/or observed scour conditions are termed scour-critical bridges. There are approximately 17,000 scour-critical bridges in the United States. This designation comes in part from the use of overly conservative methods that predict excessive scour depths in erosion-resistant materials. Other methods capable of overcoming this overconservatism are relatively uneconomical because they require site-specific erosion testing. This paper proposes a new bridge scour assessment method. The new method, termed the observation method for scour (OMS), was developed for the Texas Department of Transportation’s statewide bridge scour assessment program. The proposed method does not require site-specific erosion testing and accounts for time-dependent scour in erosion-resistant materials. OMS was developed for use as a first-order assessment in combination with a routine bridge inspection ...

Simplified Method for Estimating Scour at Bridges

This research proposes a new method to assess a bridge for scour. It is made up of three levels of assessments. The first level is termed Bridge Scour Assessment 1 (BSA 1). The second and third levels are termed BSA 2 and BSA 3, respectively. BSA 1 overcomes the qualitative nature of current initial evaluation procedures by extrapolating present scour measurements to obtain the scour depth corresponding to a specified future flood event. It utilizes computer-generated extrapolation charts based on a large combination of hypothetical bridges, which relate the future scour depth/maximum observed scour depth ratio to the future flood velocity/maximum observed flood velocity ratio. BSA 2 has to be carried out if BSA 1 does not conclude with a specific plan of action for the bridge. BSA 2 determines the maximum scour depth. Though conservative, BSA 2 was introduced due to its simplicity. BSA 3 has to be carried out if BSA 2 does not conclude with a specific plan of action. BSA 3 involves the calculation of time-dependent scour depth rather than simply using the maximum scour depth. BSA 3 is valuable in the case of highly erosion-resistant materials that do not achieve the maximum scour depth within the lifetime of a bridge. Both BSA 2 and BSA 3 utilize erosion classification charts that replace site-specific erosion testing for preliminary evaluations. The scour vulnerability depends on the comparison of the predicted scour depth and the allowable scour depth of the foundation. Hydrologic and hydraulic computer programs were developed to obtain the flow parameters. These programs generate maps of the maximum previous flood recurrence interval experienced by a specified bridge in Texas and converts flow into flow velocities. The 11 case histories used as validation showed good agreement between predicted and measured values. BSA 1 was then applied to 16 bridges. In this process, 6 out of 10 scour-critical bridges were found to be stable in terms of scour. The proposed bridge scour assessment procedure allows for the economical and relatively simple evaluation of scour-critical bridges. It also overcomes the over-conservatism in current methods.

Best-fit distribution and log-normality for tsunami heights along coastal lines

The best-fit distribution of the tsunami height was investigated along the Eastern Coast of Korean Peninsula. Firstly, the tsunami heights corresponding to the nine probable undersea earthquakes were obtained along the coastline using the numerical simulation. The method of L-moment ratio diagram was used to identify the best-fit probability density function of the tsunami heights caused by each undersea earthquake. The result indicates the generalized Pareto distribution is the best-fit distribution representing the tsunami heights regardless of the characteristics of the undersea earthquakes. This is particularly because the area of high tsunami heights and its relative magnitude to the adjacent locations were similar for the most simulations cases. In addition, this study further investigated the reason why the tsunami height distribution is not represented by the log-normal (LN) distribution as suggested by the previous studies. Result of the investigation indicates that the log-normality of the tsunami heights can be preserved when the length of a coastal line is not long such that the homogeneity of the length of the wave propagation paths reaching at different locations of the coastal line is preserved. This subsequently secures the central limit theorem making the distribution of the tsunami heights have the LN distribution. As the length of the coastal line increases, the deviation of the tsunami height distribution from the log-normality increases.

Drought Risk Analysis Using Stochastic Rainfall Generation Model and Copula Functions

This study performed the bivariate drought frequency analysis for duration and severity of drought, using copula functions which allow considering the correlation structure of joint features of drought. We suggested the confidence intervals of duration-severity-frequency (DSF) curves for the given drought duration using stochastic scheme of monthly rainfall generation for 57 sites in Korea. This study also investigated drought risk via illustrating the largest drought events on record over 50 and 100 consecutive years. It appears that drought risks are much higher in some parts of the Nakdong River basin, southern and east coastal areas. However, such analyses are not always reliable, especially when the frequency analysis is performed based on the data observed over relatively short period of time. To quantify the uncertainty of drought frequency curves, the droughts were filtered by different durations. The 5%, 25%, 50%, 75%, and 95% confidence intervals of the drought severity for a given duration were estimated based on the simulated rainfall time series. Finally, it is shown that the growing uncertainties is revealed in the estimation of the joint probability using the two marginal distributions since the correlation coefficient of two variables is relatively low.

Let-It-Rain: a web application for stochastic point rainfall generation at ungaged basins and its applicability in runoff and flood modeling

We present a web application named Let-It-Rain that is able to generate a 1-h temporal resolution synthetic rainfall time series using the modified Bartlett–Lewis rectangular pulse (MBLRP) model, a type of Poisson stochastic rainfall generator. Let-It-Rain, which can be accessed through the web address http://www.LetItRain.info, adopts a web-based framework combining ArcGIS Server from server side for parameter value dissemination and JavaScript from client side to implement the MBLRP model. This enables any desktop and mobile end users with internet access and web browser to obtain the synthetic rainfall time series at any given location at which the parameter regionalization work has been completed (currently the contiguous United States and Republic of Korea) with only a few mouse clicks. Let-It-Rain shows satisfactory performance in its ability to reproduce observed rainfall mean, variance, auto-correlation, and probability of zero rainfall at hourly through daily accumulation levels. It also shows a reasonably good performance in reproducing watershed runoff depth and peak flow. We expect that Let-It-Rain can stimulate the uncertainty analysis of hydrologic variables across the world.

Mississippi River Levee Failures: June 2008 Flood

During the spring and summer of 2008, record rainfall in the Midwest United States led to severe flooding as water overtopped the levees bordering the Mississippi River and its tributaries. The erosion associated with the overtopping resulted in levee breaches in many places. After the flooding, a field reconnaissance team was sent to collect time-sensitive data and provide a comprehensive overview of the performance of the levees during the flooding. Two locations where levee overtopping occurred are particularly interesting because of their differing site conditions and performance. This paper presents the levee overtopping case histories of the Winfield-Pin Oak site which was overtopped and severe erosion led to failure, and the Brevator site which was also overtopped but did not fail. Included are a hydrological investigation, documented site conditions, geotechnical soil properties, a soil erodibility analysis, and the documented levee vegetative cover. Levee performance is influenced by the flood conditions, the site conditions, and the soil properties. Both sites in this study experienced large levels and durations of overtopping water, but it is proposed that the Brevator site survived because of its vegetative cover and more erosion resistant soils. Erosion is a very complicated phenomenon that cannot be described by any one parameter, but in all cases, dense and consistent native vegetative cover can greatly improve the overall levee performance.

Application of the Poisson Cluster Rainfall Generation Model to the Urban Flood Analysis

This study examined the applicability of MBLRP (Modified Bartlett-Lewis Rectangular Pulse) rainfall generation model for an urban flood simulation which is a type of Poisson cluster rainfall generation model. This study constructed XP-SWMM model for Namgajwa area of Hongjecheon basin, which is a two-dimensional pipe network-surface flood simulation program and computed a flood discharge and a flooded area with input data of synthetic rainfall time series of 200 years that were generated by the MBLRP model. This study compared the data of flood with synthetic rainfall and flood with corresponding values which were based on design rainfall. The results showed that the flooded area computed with MBLRP model was somewhat smaller than the corresponding values on the basis of the design. A degree of underestimation was from 8% (5 year) to 34% (200 year) and the degree of underestimation increased as a return period increased. This study is meaningful in that it proposes methodology that enables quantifiability of uncertain variables which are related to a flooding through Monte Carlo analysis of urban flooding simulation and applicability and limitations thereof.

Applicability of a Space-time Rainfall Downscaling Algorithm Based on Multifractal Framework in Modeling Heavy Rainfall Events in Korean Peninsula

본 연구에서는 강우의 시공간적 멀티프랙탈 특성에 기반을 둔 다운스케일링 알고리즘(RDSTMF-Rainfall Downscaling in Space-Time Multifractal Framework)을 한반도에 적용하여 그 적용성을 살펴보았다. 이를 위하여 2008년부터 2012년까지 우리나라에 호우주의보를 일으킨 8개의 이벤트에 대한 레이더강우자료를 분석하여...

A Poisson Cluster Stochastic Rainfall Generator That Accounts for the Interannual Variability of Rainfall Statistics: Validation at Various Geographic Locations across the United States

A novel approach for a Poisson cluster stochastic rainfall generator was validated in its ability to reproduce important rainfall and watershed response characteristics at 104 locations in the United States. The suggested novel approach, The Hybrid Model (THM), as compared to the traditional Poisson cluster rainfall modeling approaches, has an additional capability to account for the interannual variability of rainfall statistics. THM and a traditional approach of Poisson cluster rainfall model (modified Bartlett-Lewis rectangular pulse model) were compared in their ability to reproduce the characteristics of extreme rainfall and watershed response variables such as runoff and peak flow. The results of the comparison indicate that THM generally outperforms the traditional approach in reproducing the distributions of peak rainfall, peak flow, and runoff volume. In addition, THM significantly outperformed the traditional approach in reproducing extreme rainfall by 2.3% to 66% and extreme flow values by 32% to 71%.